1. Field of the Invention
This invention relates to an apparatus for protecting self turn-off switching devices contained in inverters, chopper circuits or the like, and more particularly to a snubber energy recovery circuit for recovering energy from a snubber circuit that has stored energy caused by the switching of self turn-off switching devices.
2. Description of the Related Art
FIG. 23 shows the construction of a chopper circuit with a conventional snubber circuit and recovery circuit. In the chopper circuit, the positive terminal of a direct-current (DC) power supply Vd is connected to one end of a load 1 via a reactor La and a switching device with a self gate turn-off function (hereinafter, referred to as a GTO), and its negative terminal is connected to the other end of the load 1, across which a freewheeling diode Df is connected.
Connected in parallel with the GTO is a snubber circuit composed of a series circuit that contains a snubber diode Ds and a snubber capacitor Cs. The junction point of the snubber diode Ds and snubber capacitor Cs is connected to the positive terminal of the DC power source Vd via a series circuit that contains a recovery diode Do, a reactor Lo, and an auxiliary power supply Eo. A conventional snubber energy recovery circuit is composed of the above-mentioned snubber circuit and recovery circuit.
In the chopper circuit of FIG. 23, when the GTO turns on, the load 1 will be applied with the voltage Vd, which will increase the load current IL. When the GTO turns off, this will cause VL=0, which will allow current IL flowing in the load 1 to flow through the freewheeling diode Df until it attenuates.
The snubber circuit has the function of absorbing surge voltages caused by the reactor La, inductance components of the wires, etc, when the GTO turns off. Specifically, if no snubber circuit is used, when the GTO is turned off, the load current IL will be allowed to circulate through the freewheeling diode Df as noted earlier, but the current flowing through the reactor La will have nowhere to go, with the result that an excessive voltage will be applied to the GTO, thus destroying the GTO.
If a snubber circuit is connected to the GTO, when the GTO is turned off, the energy in the reactor La is stored in the capacitor Cs via the diode Ds, thereby protecting the GTO from excessive voltages and currents.
When the GTO then turns on, the voltage of the capacitor Cs discharges by means of a closed circuit made up of the recovery diode Do, reactor Lo, auxiliary power supply Eo, reactor La, GTO, and capacitor Cs.
With the above-described snubber energy recovery circuit, however, the reactor Lo inserted in the recovery circuit lengthens the discharging time of the capacitor Cs, making it impossible to use this apparatus for a GTO with a high switching frequency.
If the reactor Lo is removed from the recovery circuit, a resonance current will be caused by the capacitor Cs and reactor La, which will create the problem that a large current flows through the GTO.